High-affinity choline transporter

ABSTRACT

The present invention provides a protein having high-affinity choline transporter activity which is important physiologically, a gene encoding the protein, and a method of screening a material promoting the high-affinity choline transporter activity with the use of the same, and the like. By examining high-affinity choline uptake activity of Na + -dependent transporter cDNA deduced from the genomic sequence of a nematode ( C. elegans ) in a  Xenopus  oocyte expression system, the cDNA (cho-1) of nematode high-affinity choline transporter is identified. Then the cDNA (CHT1) of rat high-affinity choline transporter is cloned from rat spinal cord by using the homology of a base sequence to this cDNA as an index. Similarly, the cDNA of human high-affinity choline transporter is cloned from human genome.

TECHNICAL FIELD

This invention relates to a protein having high-affinity cholinetransporter activity, a gene encoding said protein and the use of thesame.

PRIOR ART

The autonomic nervous system which spreads to organs throughout a bodyand regulates the most basic functions of living organism includingenergy metabolism, circulation, respiration and reproduction along withendocrine system, is classified into the sympathetic and parasympatheticnervous systems. All autonomic nerve fibers excluding postganglionicfibers of the sympathetic nerve, motor nerve fiber, and sudoriferousgland/blood vessel dilative fiber in the sympathetic nerve arecholinergic, and acetylcholine is vital for the function of theautonomic nerve and the motor nerve. It has been known that thecholinergic neuron, being observed also in the brain, is important forrecognizing function of the brain and that it degenerates after theonset of Alzheimer's disease. In the cholinergic neuron, because of lackof biosynthetic ability for choline, choline, an acetylcholinedecomposition product, is taken up into a cell by a high-affinitycholine transporter at the presynaptic terminals to be reused forsynthesizing acetylcholine. The high-affinity choline uptake is arate-limiting step for acetylcholine synthesis and is presumed toregulate the efficiency of synaptic transmission (J. Neurochem. 18,781-798, 1971, Science 178, 626-628, 1972, Biochem. Biophys. Acta 291,564-575, 1973, Mol. Pharmacol. 9, 630-639, 1973, J. Pharmacol. Exp.Ther. 192, 86-94, 1975, J. Neurochem. 30, 15-21, 1978, J. Neurochem. 44,11-24, 1985, J. Neurochem. 60, 1191-1201, 1993, J. Neurochem. 20,581-593, 1973, Eur. J. Pharmacol. 102, 369-370, 1984). To date, most ofcDNAs of transporters for major neurotransmitters have been isolated,however, a cDNA of the high-affinity choline transporter, which isphysiologically important, has not been identified.

DISCLOSURE OF THE INVENTION

So far, the existence of a protein being localized in the cholinergicneuron and having a function of taking up choline, a precursor ofacetylcholine, into a cell has been expected, but molecular propertiesof said protein, a high-affinity choline transporter, have been unknown.An object of the present invention is to provide a physiologicallyimportant protein having the high-affinity choline transporter activity,a gene which encodes the protein, and a screening method of ahigh-affinity choline transporter activity promoter using the protein,the gene and the like.

The inventors have conducted intensive study to attain theabove-mentioned object: with information of genomic project (Science282, 2012-2018, 1998), Na⁺-dependent transporter cDNAs being expectedfrom the genomic sequence of a nematode (C. elegans) were cloned one byone, and the high-affinity choline uptake activity of each cDNA wasexamined in the oocyte expression system of Xenopus, and the cDNA ofnematode high-affinity choline transporter (cho-1) was identified on thebasis of the above examination, then homologous molecules (CHT1) werecloned from rat spinal cord by using the homology of a base sequence tothe cDNA as an index. This CHT1 had no homology to neurotransmittertransporters (J. Neurochem. 71, 1785-1803, 1998), but had 20 to 25%homology to molecules which belong to Na⁺-dependent glucose transporterfamily (Nature 330, 379-381, 1987).

Northern blot analysis revealed that transcripts of CHT1 were confirmedonly in spinal cord, basal forebrain, corpus striatum and brain stem,and CHT1 seemed to be expressed in cholinergic neurons. Accordingly,CHT1 was expressed in oocytes of Xenopus. As a result, choline uptakeactivity that is Na⁺-dependent and completely inhibited byhemicholinium-3 was observed. These results indicate that CHT1 hashigh-affinity choline transporter activity. Further, the inventors havecloned choline transporter cDNAs derived from a human and from a mouse,and determined their base sequences, and have confirmed that theirexpression products have high-affinity choline uptake activity. Thepresent invention has thus completed.

The present invention relates to a gene which encodes a protein havinghigh-affinity choline transporter activity, a gene which encodes aprotein (a) or (b) described below; (a) a protein comprising an aminoacid sequence represented by Seq. ID No. 2, (b) a protein comprising anamino acid sequence where one or a few amino acids are deficient,substituted or added in the amino acid sequence represented by Seq. IDNo.2, and having high-affinity choline transporter activity, DNAcontaining a base sequence represented by Seq. ID No. 1 or itscomplementary sequence and a part or a whole of these sequences, DNAderived from a nematode which hybridizes with DNA comprising a geneaccording to the above under a stringent condition, and encodes aprotein having high-affinity choline transporter activity, a gene whichencodes a protein (a) or (b) described below; (a) a protein comprisingan amino acid sequence represented by Seq. ID No. 4, (b) a proteincomprising an amino acid sequence where one or a few amino acids aredeficient, substituted or added in the amino acid sequence representedby Seq. ID No.4, and having high-affinity choline transporter activity,DNA containing a base sequence represented by Seq. ID No. 3 or itscomplementary sequence and a part or a whole of these sequences, DNAderived from a rat which hybridizes with DNA comprising a gene accordingto the above under a stringent condition, and encodes a protein havinghigh-affinity choline transporter activity, a gene which encodes aprotein (a) or (b) described below; (a) a protein comprising an aminoacid sequence represented by Seq. ID No. 6, (b) a protein comprising anamino acid sequence where one or a few amino acids are deficient,substituted or added in the amino acid sequence represented by Seq. IDNo.6, and having high-affinity choline transporter activity, DNAcontaining a base sequence represented by Seq. ID No. 5 or itscomplementary sequence and a part or a whole of these sequences, DNAderived from a human which hybridizes with DNA comprising a genecontaining Seq ID No. 5 or its complementary sequence or a portion ofeither under a stringent condition, and encodes a protein havinghigh-affinity choline transporter activity, a gene which encodes aprotein (a) or (b) described below; (a) a protein comprising an aminoacid sequence represented by Seq. ID No. 8, (b) a protein comprising anamino acid sequence where one or a few amino acids are deficient,substituted or added in the amino acid sequence represented by Seq. IDNo.8, and having high-affinity choline transporter activity, DNAcontaining a base sequence represented by Seq. ID No. 7 or itscomplementary sequence and a part or a whole of these sequences, and DNAderived from a mouse which hybridizes with DNA comprising a genecomprising a base sequence represented by Seq ID No. 7 or itscomplementary sequence or a portion of either under a stringentcondition, and encodes a protein having high-affinity cholinetransporter activity.

The present invention also relates to a protein having high-affinitycholine transporter activity, a protein comprising an amino acidsequence represented by Seq. ID No. 2, a protein comprising an aminoacid sequence where one or a few amino acids are deficient, substitutedor added in the amino acid sequence represented by Seq. ID No.2, andhaving nematode high-affinity choline transporter activity, a proteincomprising an amino acid sequence represented by Seq. ID No. 4, aprotein comprising an amino acid sequence where one or a few amino acidsare deficient, substituted or added in the amino acid sequencerepresented by Seq. ID No.4, and having rat high-affinity cholinetransporter activity, a protein comprising an amino acid sequencerepresented by Seq. ID No. 6, a protein comprising an amino acidsequence where one or a few amino acids are deficient, substituted oradded in the amino acid sequence represented by Seq. ID No.6, and havinghuman high-affinity choline transporter activity, a protein comprisingan amino acid sequence represented by Seq. ID No. 8, and a proteincomprising an amino acid sequence where one or a few amino acids aredeficient, substituted or added in the amino acid sequence representedby Seq. ID No.8, and having mouse high-affinity choline transporteractivity.

The present invention further relates to a fusion protein beingconstructed by expressing a cDNA encoding fusion proteins of a proteinhaving high-affinity choline transporter activity and a marker proteinand/or a peptide tag, the fusion protein according to the above, whereinthe protein having high-affinity choline transporter activity hasnematode high-affinity choline transporter activity as described asabove, the fusion protein according to the above, wherein the proteinhaving high-affinity choline transporter activity has rat high-affinitycholine transporter activity, the fusion protein according to the above,wherein the protein having high-affinity choline transporter activityhas human high-affinity choline transporter activity, and the fusionprotein according to the above, wherein the protein having high-affinitycholine transporter activity has mouse high-affinity choline transporteractivity.

The present invention still further relates to an antibody whichspecifically binds to a protein having high-affinity choline transporteractivity, the described antibody wherein the protein havinghigh-affinity choline transporter activity has nematode high-affinitycholine transporter activity, the described antibody, wherein theprotein having high-affinity choline transporter activity has rathigh-affinity choline transporter activity, the described antibody,wherein the protein having high-affinity choline transporter activityhas human high-affinity choline transporter activity, the describedantibody, wherein the protein having high-affinity choline transporteractivity has mouse high-affinity choline transporter activity, and thedescribed antibody according to any of the above, wherein the antibodyis a monoclonal antibody.

The present invention also relates to a host cell containing anexpression system which can express a protein having high-affinitycholine transporter activity, the host cell, wherein the protein havinghigh-affinity choline transporter activity has nematode high-affinitycholine transporter activity, the host cell wherein the protein havinghigh-affinity choline transporter activity has rat high-affinity cholinetransporter activity, the host cell, wherein the protein havinghigh-affinity choline transporter activity has human high-affinitycholine transporter activity, and the host cell, wherein the proteinhaving high-affinity choline transporter activity has mousehigh-affinity choline transporter activity.

The present invention further relates to a non-human animal in whichfunction of a gene which encodes a protein having high-affinity cholinetransporter activity is deficient or overexpresses on its chromosome,the non-human animal, wherein the protein having high-affinity cholinetransporter activity has nematode high-affinity choline transporteractivity, the non-human animal, wherein the protein having high-affinitycholine transporter activity has rat high-affinity choline transporteractivity, the non-human animal, wherein the protein having high-affinitycholine transporter activity has human high-affinity choline transporteractivity, the non-human animal wherein the protein having high-affinitycholine transporter activity has mouse high-affinity choline transporteractivity, and the non-human animal according to any of the above,wherein the non-human animal is a mouse or a rat.

The present invention still further relates to a preparing method of acell having high-affinity choline transporter activity characterized inintroducing the gene or the DNA as described above into a cell whosefunction of a gene which encodes a protein having high-affinity cholinetransporter activity is deficient on its chromosome, the preparingmethod of a cell having high-affinity choline transporter activity,wherein the cell having high-affinity choline transporter activity isintegrated with the gene or the DNA as described above in itschromosome, and stably shows high-affinity choline transporter activity,and a cell having high-affinity choline transporter activity beingobtainable by the preparing method of a cell having high-affinitycholine transporter activity as described.

The present invention also relates to a screening method of a promoteror a suppressor of high-affinity choline transporter activitycharacterized in measuring/evaluating high-affinity choline transporteractivity of the protein having high-affinity choline transporteractivity according to the above in the presence of a subject material, ascreening method of a promoter or a suppressor of high-affinity cholinetransporter activity, or of high-affinity choline transporter expressioncharacterized in comprising the steps of: a cell membrane or a cellwhich expresses a protein having high-affinity choline transporteractivity is cultivated in vitro in the presence of a subject material;the activity and/or the expression amount of a protein havinghigh-affinity choline transporter activity in the cell membrane or thecell is measured/evaluated, the screening method of a promoter or asuppressor of high-affinity choline transporter activity, or ofhigh-affinity choline transporter expression, wherein the cell membraneor the cell which expresses a protein having high-affinity cholinetransporter activity is the host cell containing an expression systemwhich can express a protein having high-affinity choline transporteractivity according to any of the above, or is the cell havinghigh-affinity choline transporter activity according to the above, thescreening method of a promoter or a suppressor of high-affinity cholinetransporter activity, or of high-affinity choline transporter expressionaccording to the above, wherein the protein having high-affinity cholinetransporter activity is a recombinant protein, a screening method of apromoter or a suppressor of high-affinity choline transporter activity,or of high-affinity choline transporter expression characterized incomprising the steps of: a cell obtained from the non-human animalaccording to any of the above is cultivated in vitro in the presence ofa subject material; the activity and/or the expression amount of aprotein having high-affinity choline transporter activity in the cell ismeasured/evaluated, a screening method of a promoter or a suppressor ofhigh-affinity choline transporter activity, or of high-affinity cholinetransporter expression characterized in administering a subject materialto a non-human animal and then evaluating the activity and/or theexpression amount of a protein having high-affinity choline transporteractivity, a screening method of a promoter or a suppressor ofhigh-affinity choline transporter activity, or of high-affinity cholinetransporter expression characterized in administering a subject materialto a non-human animal whose function of a gene encoding a protein havinghigh-affinity choline transporter activity is deficient or overexpresseson its chromosome, and then evaluating the activity and/or theexpression amount of a protein having high-affinity choline transporteractivity, a screening method of a promoter or a suppressor ofhigh-affinity choline transporter activity, or of high-affinity cholinetransporter expression characterized in administering a subject materialto a non-human animal whose function of a gene encoding a protein havinghigh-affinity choline transporter activity is deficient or overexpresseson its chromosome, and then evaluating the activity and/or theexpression amount of a protein having high-affinity choline transporteractivity in comparison with the case using wild-type non-human animal,and the screening method of a promoter or a suppressor of high-affinitycholine transporter activity, or of high-affinity choline transporterexpression according to any of the above, wherein the non-human animalis a mouse or a rat.

The present invention further relates to a material which promotesactivity or expression of a protein having high-affinity cholinetransporter activity being obtainable by the screening method asdescribed above, a material which suppresses activity or expression of aprotein having high-affinity choline transporter activity beingobtainable by the screening method according to the above, a medicalconstituent characterized in being used for a medical treatment for apatient who needs elevation of the activity or enhancement of theexpression of a high-affinity choline transporter, and containing theprotein according to any of the above, and/or the material whichpromotes activity or expression of a protein having high-affinitycholine transporter activity according to the above as an activecomponent, and a medical constituent characterized in being used formedical treatment for a patient who needs suppression of the activity orthe expression of a high-affinity choline transporter, and containingthe protein according to the above, and/or the material which suppressesthe activity or the expression of a protein having high-affinity cholinetransporter activity according to the above as an active component.

The present invention still further relates to a diagnostic method fordiseases relating to the expression or the activity of a high-affinitycholine transporter characterized in comparing a DNA sequence encoding ahigh-affinity choline transporter in a sample to a DNA sequence encodingthe protein as described above, a diagnostic probe for Alzheimer'sdisease comprising a whole or a part of an antisense strand of DNA orRNA encoding the protein as described above, and a diagnostic drug forAlzheimer's disease characterized in containing the diagnostic probe asdescribed above and/or the antibody as described above.

BRIEF EXPLANATION OF THE DRAWINGS

The patent or application file contains at least one drawing executed incolor. Copies of this patent or patent application publication withcolor drawing(s) will be provided by the Office upon request and paymentof the necessary fee.

FIG. 1 is a view showing the result of [³H] choline uptake of oocytesfrom Xenopus of the present invention being injected with nematode cho-1(C48D1.3 cRNA) or water.

FIG. 2 is a view showing the result of the effect of Na⁺ on cholineuptake of oocytes from Xenopus of the present invention being injectedwith nematode cho-1 (C48D1.3 cRNA) or water.

FIG. 3 is a view showing the result of the HC3-induced inhibition ofcholine uptake of oocytes from Xenopus of the present invention beinginjected with nematode cho-1 (C48D1.3 cRNA) or water.

FIG. 4 is a view showing amino acid sequences of rat CHT1 and nematodeCHO-1 of the present invention respectively.

FIG. 5 is a view showing the distribution of neurons expressingcho-1::gfp of the present invention in the nervous system of nematode.

FIG. 6 is a view showing the phylogenetic tree of Na⁺-dependent glucosetransporter family.

FIG. 7 is a view showing an expected topology of rat CHT1 of the presentinvention.

FIG. 8 is a view showing the result of Northern blot analysis of CHT1mRNA transcript in rat tissue of the present invention.

FIG. 9 is a view showing the result of in situ hybridization analysis ofCHT1 transcript in a rat brain of the present invention.

FIG. 10 is a view showing the result of in situ hybridization analysisof CHT1 transcript in a spinal cord of the present invention.

FIG. 11 is a view showing the result of [³H] choline uptake of oocytesfrom Xenopus of the present invention being injected CHT1 cRNA of thepresent invention or water.

FIG. 12 is a view showing the effect of choline concentration on cholineuptake in CHT1 of the present invention.

FIG. 13 is a view showing the result of HC3-induced inhibition ofcholine uptake of CHT1 of the present invention.

FIG. 14 is a view showing the result of Na⁺- and Cl⁻-dependent cholineuptake of CHT1 of the present invention.

FIG. 15 is a view showing the result of [³H] HC3 binding to the membraneprepared from COS7 cells being introduced with CHT1 cDNA of the presentinvention or vector pcDNA 3.1 separately.

FIG. 16 is a view showing the result of saturation analysis of specific[³H] HC3 binding to the membrane prepared from COS7 cells beingintroduced with CHT1 cDNA of the present invention or vector pcDNA 3.1separately.

FIG. 17 is a view showing the result of displacement of specific [³H]HC3 binding by HC3 of the present invention, choline (Cho),acetylcholine (ACh).

BEST MODE FOR CARRYING OUT THE INVENTION

The cDNA of nematode high-affinity choline transporter of the presentinvention, being described in Seq. ID No. 1, can be obtained byinjecting each cRNA prepared from candidate full-length cDNAs, which areexpected as a member of Na⁺-dependent transporter family according to C.elegans genome project, into oocytes of Xenopus, and examining theuptake of choline. The high-affinity uptake of choline in brainsynaptosomes of mammals was completely inhibited by 1 μM hemicholinium-3(HC3) (Ki=10-100 nM), while the low-affinity uptake of choline, which isdistributed in every cells, was inhibited only by HC3 with higherconcentration (Ki=50 μM). Therefore, the sensitivity to 1 μM HC3 can beused as criteria of high-affinity choline uptake during the process. Forexample, it is possible to confirm the identification, the expression,and the localization of an object gene from the candidate cDNA of anematode (C. elegans) as follows.

It has been found that cDNA corresponding to the gene expected asC48D1.3 promotes significant choline uptake, being inhibited by 1 μMHC3, in the high-affinity choline uptake process. FIG. 1 shows theresult of [³H] choline uptake of oocytes from Xenopus being injectedwith C48D1.3 cRNA or water. In FIG. 1, the closed and the open columnsindicate choline uptake in the absence or the presence of 1 μM HC3respectively, and each column is shown by mean ±SEM (n=6 to 8 oocytes).FIG. 2 shows the effect of Na⁺ on the choline uptake, and the closedcolumns indicate choline uptake measured in the standard solution([Na⁺]=100 mM), the open columns indicate choline uptake in the absenceof Na⁺ (Na⁺ was substituted with Li⁺). In addition, FIG. 3 shows theinhibition of choline uptake induced by HC3. Based on theabove-mentioned FIGS. 2 and 3, it is presumed that the uptake isNa⁺-dependent, and that Ki of HC3 is 50 nM. The cDNA clone wasdesignated as cho-1 (high-affinity choline transporter-1).

By comparing a base sequence of cDNA and that of genome, cho-1 gene wasfound to comprise 9 exons. A protein expected from a base sequence ofcDNA of cho-1 includes 576 amino acid residues (see FIG. 4), and thisprotein, being represented by Seq. ID No. 2, can be constructed by ausual method. When the available data base was searched, the amino acidsequences of cho-1 showed weak, but significant homology to members ofNa⁺-dependent glucose transporter family. Hydrophobic analysis andcomparison to other transporters suggest that there is atwelve-transmembrane region (see FIG. 7).

Then, in order to identify cells expressing cho-1 in the nervous systemof a nematode (C. elegans), a gene of a green fluorescent protein (GFP)fused with a region 5.1 kb upstream from cho-1 gene was introduced intoa nematode, and distribution of neurons expressing cho-1::gfp wasexamined. A photograph of L1 larva possessing cho-1::gfp reporter DNA atthe outside of chromosome is shown as FIG. 5 (scale bar; 50 μm). In FIG.5, the arrowhead indicates nerve ring. In the ventral nerve cord, GFP isexpressed only in cholinergic motor nerve, however, some of DA, DB nervecells do not express GFP owing probably to deficiency of reporter DNA atthe outside of chromosome. It supports the idea that cho-1 is ahigh-affinity choline transporter of the cholinergic neuron.

The cDNA of rat high-affinity choline transporter of the presentinvention, being described in Seq. ID No. 3, can be prepared, forexample, by a method comprising the steps of: paying attention to cho-1homologous molecules of vertebrates and searching data base with aminoacid sequences expected from cho-1, and identifying one candidate(GenBank accession number: AQ 316435) in human genomic survey sequence(GSS); amplifying cDNA fragments from rat spinal cord cDNA by PCR withdegenerate primers on the basis of homology of base sequences betweenthe human genome DNA and cho-1; screening rat spinal cord cDNA librarywith this fragment, and a positive cDNA clone was obtained. A proteinwith 580 amino acid residues showing 51% identity and 70% similarity tocho-1 was expected from the base sequence of the longest reading frame(see FIG. 4). This rat cDNA clone was designated as CHT1. In FIG. 4,each amino acid sequence of rat CHT1 and nematode CHO-1 is shown, andthe identical and the similar residues are indicated on a black groundand a gray ground respectively. The expected transmembrane region I-XIIis underlined. This protein represented by Seq. ID No. 4 can beconstructed by a usual method.

The above-mentioned amino acid sequence of CHT1 is significantlyhomologous to members of Na⁺-dependent glucose transporter family (20 to25%). The phylogenetic tree of Na⁺-dependent glucose transporter familymade by neighbor-joining method using a program CLUSTALW of NationalInstitute of Genetics (Mishima, Japan) is shown in FIG. 6. In FIG. 6,the percentage of the identical amino acids, being contained in eachprotein, to rat CHT1 is shown on the right side. On the other hand, nohomology was observed to a yeast choline transporter (J. Biol. Chem.265, 15996-16003, 1990), a creatine transporter which had beenoriginally reported as a high-affinity choline transporter (Biochem.Biophys. Res. Commun. 198, 637-645, 1994), and other neurotransmittertransporters.

The expected topology of CHT1 is thought to be the same as that ofnematode CHO-1 fundamentally. FIG. 7 shows the expected topology of ratCHT1. In FIG. 7, the closed circles indicate the identical residues, theshadowed circles indicate highly conserved residues, and open circlesindicate nonsimilar residues. The offshoots indicate the expectedglycosylation sites. P among the circles shows the expected parts ofphosphorylation induced by protein kinase C.

Next, the distribution of CHT1 mRNA expression was examined by Northernblot analysis and in situ hybridization. The expression of transcriptswith the length of about 5 kb was confirmed by Northern blot analysis ofvarious tissues of rats. FIG. 8 shows the result of Northern blotanalysis of mRNA transcript of CHT1 in rat tissue, and the length of RNAstandard (0.24 to 9.5 kb; GIBCO BRL) is exhibited on the left side. Asshown in FIG. 8, an abundance of transcripts were confirmed in basalforebrain, brain stem and spinal cord, and a little of those wereconfirmed in corpus striatum. These tissues are known to containcholinergic neurons. On the other hand, no transcript was observed inother regions of the brain or in tissues of non-nervous systems.

Consistent with these results, in situ hybridization confirmed theexpression of CHT1 mRNA in cell groups of main cholinergic neuronsincluding corpus striatum, cell population in basal forebrain and ventalhorn in spinal cord. FIGS. 9 and 10 (scale bar; 1 mm) show micrographsof sections in bright-field, which were hybridized with a cRNA probe ofan antisense labeled by digoxigenin. These micrographs relate to in situhybridization analysis of CHT1 transcripts in rat brain and spinal cord.FIG. 9 indicates that mRNA transcripts of CHT1 were detected in verticaland horizontal limbs of the diagonal band (VDB, HDB), medial septalnucleus (MS), caudate and putamen (Cpu), and olfactory tubercle (Tu).FIG. 10 indicates that the expression was observed in ventral horn (VH)in spinal cord. Further, the adjacent section hybridized with a probe ofvesicle acetylcholine transporter showed essentially same distribution.This expression distribution is essentially same as the reporteddistribution of cholineacetyl group transferase or vesicle acetylcholinetransporter. These results show that the expression of CHT1 mRNA islimited to cholinergic neurons.

Next, choline uptake of CHT1 was examined by using oocytes of Xenopus.The choline uptake of the oocytes injected with CHT1 cRNA was 2 times to4 times more than that of controls injected with water. FIG. 11 showsthe result of (³[H] choline uptake of oocytes of Xenopus injected withCHT1 cRNA or water. In FIG. 11, the open and the closed columnsrespectively indicate choline uptake in the standard solutionscontaining 100 mM NaCl or LiCl, and each column is shown by mean ±SEM(n=6 to 8 oocytes). The effect of choline concentration on cholineuptake is shown in FIG. 12. In FIG. 12, choline uptake of oocytesinjected with water was subtracted from that of oocytes injected withcRNA in order to figure out CHT1-induced choline uptake, and the cholineuptake was fitted to Michaelis-Menten curve. As shown in FIG. 12,choline uptake of CHT1 saturated when increasing choline concentration(Km=2.2±0.2 μM, n=3). The Km of endogenous choline uptake of control ishigher than 10 μM.

Then, the result of HC3-induced inhibition of choline uptake is shown inFIG. 13. FIG. 13 indicates that choline uptake of CHT1 is completelyinhibited by 0.1 μM HC3 (Ki=2-3 nM), whereas 10 μM HC3 induced onlyslight inhibition in control. As shown in FIG. 14, ion-dependency ofcholine uptake of CHT1 was examined and found to be Cl⁻-dependent aswell as Na⁺-dependent. The closed and the open columns indicate cholineuptake of oocytes injected with water and with cRNA respectively (100 mMNaCl in the standard solution is substituted with 100 mM of each salt)shown in the figure. These results indicate that CHT1 has thecharacteristics expected from high-affinity choline uptake in brainsynaptosomes (high-affinity to choline, high sensitivity to HC3, andNa⁺—Cl⁻-dependency) (J. Neurochem. 27, 93-99, 1976).

In addition, [³H] HC3 binding activity of membranes prepared from COS7cells introduced with CHT1 cDNA and a vector (control) respectively wasexamined. The result is shown in FIG. 15. As FIG. 15 indicates,Na⁺-dependent [³H] HC3 binding was observed in a membrane of a cellwhere CHT1 was expressed, but not in a control membrane. Subsequently, asaturation analysis was conducted for specific [³H] HC3 binding. Asshown in FIG. 16, equilibrium dissociation constant (Kd) was estimatedto be 1.6±0.2 μM(n=3). This value was similar to that reported in brainsynaptosomes (J. Neurochem. 60, 1191-1201, 1993, Life Sci. 35,2335-2343, 1984, Brain Res. 348, 321-330, 1985). Further, displacementof specific [³H] HC3 binding by HC3, choline (Cho) and acetylcholine(Ach) was examined. Acetylcholine was measured in the presence of 1 μMphysostigmine. The result is shown in FIG. 17. FIG. 17 indicates thatspecific [³H] HC3 binding was displaced when the concentration ofcholine was at least about 10 times lower than that of acetylcholine.These results show that CHT1 is a HC3 binding site as well as ahigh-affinity choline transporter.

The cDNA of human high-affinity choline transporter of the presentinvention, being represented by Seq. ID No.5, can be prepared, forexample, as follows: data base search was conducted with the amino acidsequence of nematode (C. elegans) CHO-1 to find a sequence of specifichuman genome DNA fragment having significant homology (R-107P12, a cloneof human genomic survey sequence; GenBank accession number: AQ316435); agene-specific primers for PCR were designed based on a base sequence ofsaid DNA fragment; 5′-RACE (rapid amplification of cDNA ends) and3′-RACE were conducted using Marathon-Ready™ cDNA (Clontech) of humanwhole brain, together with an attached adapter primer; the obtained PCRproduct was cloned into a cloning vector for PCR, and a base sequence ofinserted DNA was determined. In addition, an amino acid sequenceexpected from this DNA sequence is represented by Seq. ID No. 6. Aprotein having human high-affinity choline transporter activityrepresented by said Seq. ID No. 6 can be constructed by a usual methodon the basis of DNA sequence information shown in Seq. ID No. 5.

The cDNA of mouse high-affinity choline transporter of the presentinvention, being represented by Seq. ID No.7, can be prepared, forexample, as follows: data base search was conducted with the amino acidsequence of nematode (C. elegans) CHO-1 to find a sequence of specifichuman genome DNA fragment having significant homology (R-107P12, a cloneof human genomic survey sequence; GenBank accession number: AQ316435); agene-specific primer for PCR was designed based on a base sequence ofsaid DNA fragment; 5′-RACE (rapid amplification of cDNA ends) and3′-RACE were conducted using Marathon-Ready™ cDNA (Clontech) of mousewhole brain, together with an attached adapter primer; the obtained PCRproduct was cloned into a cloning vector for PCR, and a base sequence ofinserted DNA was determined. In addition, an amino acid sequenceexpected from this DNA sequence is represented by Seq. ID No. 8. Aprotein having mouse high-affinity choline transporter activityrepresented by said Seq. ID No. 8 can be constructed by a usual methodon the basis of DNA sequence information shown in Seq. ID No. 7.

Examples of a protein having high-affinity choline transporter activityof the present invention include a protein derived from naturalmaterials and a recombinant protein. In addition to the ones representedby Seq. ID Nos. 2, 4, 6 and 8, which are specifically disclosed above, aprotein comprising an amino acid sequence wherein one or a few aminoacids are deficient, substituted or added in amino acid sequencesrepresented by Seq. ID Nos. 2, 4, 6 and 8, and having high-affinitycholine transporter activity is also included. These proteins can beprepared by known methods. Further, examples of a gene or DNA encoding aprotein having high-affinity choline transporter activity of the presentinvention include, in addition to the ones represented by Seq. ID Nos.1, 3, 5 and 7, which are specifically disclosed above, a gene or DNAwhich encodes a protein comprising an amino acid sequence wherein one ora few amino acids are deficient, substituted or added in amino acidsequences represented by Seq. ID Nos. 2, 4, 6 and 8, and havinghigh-affinity choline transporter activity, and DNA which encodes aprotein hybridizing with said gene or DNA under a stringent conditionand having high-affinity choline transporter activity. These genes andDNAs can be prepared by known methods.

Cholinergic neurons play an extremely important role in learning andmemory. The damage of these neurons correlates to severity of dementia.The rate-limiting step in acetylcholine synthesis is presumed to be theuptake of choline, and its activity is controlled by neural activity orvarious kinds of stimuli. In the brains of patients who sufferAlzheimer's disease, the hyperfunction of high-affinity choline uptakeand of HC3 binding activity are observed (Trends Neurosci. 15, 117-122,1992, Ann. NY Acad. Sci. 777, 197-204, 1996, J. Neurochem. 69,2441-2451, 1997). Cloning of said gene or DNA encoding a protein havinghigh-affinity choline transporter activity and said protein havinghigh-affinity choline transporter activity is important for elucidatingthe molecular mechanism of the high-affinity choline transporter and fordeveloping new therapies for Alzheimer's disease.

The fusion protein of the present invention means a substanceconstructed by binding a protein from a nematode, a rat, a human, amouse, etc., which has high-affinity choline transporter activity, to amarker protein and/or a peptide tag. As the marker protein, anyconventionally known marker protein can be used and the specificexamples are alkaline phosphatase, Fc region of an antibody, HRP, andGFP. Conventionally known peptide tags, such as Myc tag, His tag, FLAGtag, GST tag, are exemplified as specific examples of the peptide tag ofthe present invention. Said fusion proteins can be constructed by ausual method, and are useful for the purification of a protein havinghigh-affinity choline transporter activity utilizing the affinitybetween Ni-NTA and His tag, the detection of a protein havinghigh-affinity choline transporter activity, the quantitation of anantibody to a protein having high-affinity choline transporter activity,and as a diagnostic marker for Alzheimer's disease, and aninvestigational reagent in the field concerned.

As an antibody that specifically combines with a protein havinghigh-affinity choline transporter activity of the present invention, animmunospecific antibody such as a monoclonal antibody, a polyclonalantibody, a chimeric antibody, a single stranded antibody, a humanizedantibody and the like are concretely exemplified. Though theseantibodies can be constructed by a usual method with the above-mentionedprotein having high-affinity choline transporter activity as an antigen,a monoclonal antibody is more preferable among them because of itsspecificity. Said antibody that specifically binds to a protein havinghigh-affinity choline transporter activity, such as a monoclonalantibody or the like, is useful, for instance, for the diagnosis ofAlzheimer's disease, and for elucidation of molecular mechanism of ahigh-affinity choline transporter.

An antibody to a protein having high-affinity choline transporteractivity is developed by administering fragments containing the proteinhaving high-affinity choline transporter activity or its epitope, orcells that express said protein on the surface of the membrane, toanimals (preferably excluding human) with usual protocol. For instance,a monoclonal antibody can be prepared by an arbitrary method that bringsantibodies developed by cultured materials of continuous cell line, suchas hybridoma method (Nature 256,495-497, 1975), trioma method, humanB-cell hybridoma method (Immunology Today 4, 72, 1983), andEBV-hybridoma method (MONOCLONAL ANTIBODIES AND CANCER THERAPY, pp.77-96, Alan R. Liss, Inc., 1985).

In order to develop a single stranded antibody to the above-mentionedprotein having high-affinity choline transporter activity of the presentinvention, the preparation method of single stranded antibodies (U.S.Pat. No. 4,946,778) can be applied. Further, in order to express ahumanized antibody, it is possible to use transgenic mice, othermammalian animals or the like, and to isolate and identify the clonesthat express a protein having high-affinity choline transporter activitywith the above-mentioned antibodies, and to purify the polypeptide byaffinity chromatography. An antibody to a protein having high-affinitycholine transporter activity could be used, in particular, for thediagnosis and the medical treatment of Alzheimer's disease, and thelike.

This invention relates to a host cell which contains an expressionsystem that can express said protein having high-affinity cholinetransporter activity. The gene that encodes a protein havinghigh-affinity choline transporter activity can be introduced into a hostcell by a number of methods described in many standard laboratorymanuals such as by Davis et al. (BASIC METHODS IN MOLECULAR BIOLOGY,1986), and by Sambrook et al. (MOLECULAR CLONING: A LABORATORY MANUAL,2nd Ed., Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y.,1989). Examples of those methods include calcium phosphate transfection,DEAE-dextran-mediated transfection, transvection, microinjection,cationic lipid-mediated transfection, electroporation, transduction,scrape loading, ballistic introduction and infection. Examples of thehost cells include bacterial procaryotic cells such as Escherichia coli,Streptomyces, Bacillus subtilis, Streptococcus, Staphylococcus and thelike; fungous cells such as yeast, Aspergillus and the like; insectcells such as drosophila S2, spodptera Sf9 and the like; and animal orplant cells such as L cells, CHO cells, COS cells, HeLa cells, C127cells, BALB/c3T3 cells (including mutant strains deficient indihydrofolate reductase, thymidine kinase or the like), BHK21 cells,HEK293 cells, Bowes melanoma cells and the like.

As the expression system, any expression system that can express aprotein having high-affinity choline transporter activity in a host cellwill suffice. Examples of the expression system include expressionsystems derived from chromosome, episome and virus, for example, vectorsderived from bacterial plasmid, yeast plasmid, papovavirus like SV40,vaccinia virus, adenovirus, chicken pox virus, pseudorabies virus, orretrovirus, vectors derived from bacteriophage, transposon, and thecombination of these, for instance, vectors derived from genetic factorsof plasmid and of bacteriophage such as cosmid or phagemid. Theseexpression systems may contain a regulatory sequence that acts not onlyas a promoter but also as a controller of expressions.

A host cell that contains the above-mentioned expression system, cellmembrane of said host cell, and a protein having high-affinity cholinetransporter activity which is obtainable by the cultivation of said hostcell can be used in the screening method of the present invention ashereinafter described. For example, the method of F. Pietri-Rouxel etal. (Eur. J. Biochem., 247, 1174-1179, 1997) or the like can be used asthe method to obtain cell membranes, and publicly known methodsincluding ammonium sulfate or ethanol precipitation, acid extraction,anion or cation exchange chromatography, phosphocellulosechromatography, hydrophobic-interaction chromatography, affinitychromatography, hydroxyapatite chromatography, and lectinchromatography, preferably high-speed liquid chromatography can be usedto pick up said protein having high-affinity choline transporteractivity from cell cultured material and purify it. As columns used foraffinity chromatography, in particular, there are columns to which aprotein antibody having anti-high-affinity choline transporter activityis bound, or in case that a normal peptide tag is added to saidhigh-affinity choline transporter, there are columns to which materialshaving affinity to the peptide tag are bound. Proteins havinghigh-affinity choline transporter activity can be obtained by usingthese columns.

In the present invention, said non-human animal whose function of a geneencoding a protein having high-affinity choline transporter activity isdeficient on its chromosome means a non-human animal wherein a part or awhole of a gene encoding a protein having high-affinity cholinetransporter activity on chromosome is inactivated by gene mutation suchas disruption, deficiency, substitution, etc. and function of expressinga protein having high-affinity choline transporter activity is lost. Inaddition, a non-human animal that overexpresses function of a gene thatencodes a protein having high-affinity choline transporter activity onits chromosome means a non-human animal that produces larger amount of aprotein having high-affinity choline transporter activity than awild-type non-human animal does. Though specific examples of a non-humananimal of the present invention include rodents, such as mice, rats andthe like, a non-human animal of the present invention is not limited tothese animals.

Homozygous non-human animals generated according to Mendelian ratioinclude a deficient type or an overexpression type for a protein havinghigh-affinity choline transporter activity, and their littermatewild-type, and it is possible to carry out precise comparativeexperiments in individual level by using the deficient types, theoverexpression types and the littermate wild-types of these homozygousnon-human animals at the same time. Therefore, it is preferable to useanimals of the same species, more preferably the littermates, as thewild-type non-human animals, in other words, the non-human animals beingdeficient in or overexpressing the function of a gene that encodes aprotein having high-affinity choline transporter activity on theirchromosome together in, for example, the screening hereinafter describedin the present invention. The generating method of the non-human animalsbeing deficient or overexpressing the function of a gene that encodes aprotein having high-affinity choline transporter activity on theirchromosome will be explained below, with an example of knockout mice andtransgenic mice of a protein having high-affinity choline transporteractivity.

For example, a mouse being deficient in the function of a gene thatencodes a protein having high-affinity choline transporter activity onits chromosome, in other words, a knockout mouse of a protein havinghigh-affinity choline transporter activity on its chromosome can beconstructed as follows. A gene that encodes a protein havinghigh-affinity choline transporter activity is screened by using a genefragment obtained from mouse gene library by a method like PCR. Thescreened gene that encodes a protein having high-affinity cholinetransporter activity is subcloned with a viral vector or the like, andspecified by DNA sequencing. A target vector is constructed bysubstituting a whole or a part of a gene of this clone that encodes aprotein having high-affinity choline transporter activity with pMC1 neogene cassette or the like, and by introducing a diphteria toxin Afragment (DT-A) gene, a herpes simplex virus thymidine kinase (HSV-tk)gene or other such genes into 3′-terminal side.

This constructed target vector is linearized and introduced into EScells by electroporation or the like to induce homologous recombination.The ES cells wherein homologous recombination is induced by anantibiotic such as G418, ganciclovir (GANC) or the like are selectedfrom the homologous recombinants. It is preferable to confirm whetherthe selected ES cells are the recombinants of the object by Southernblot or the like. A chimeric mouse is constructed by microinjecting aclone of the confirmed ES cells into a blastocyst of a mouse and thentransplanting the blastocyst into a recipient mouse. A heterozygousmouse can be obtained by interclossing the chimeric mouse with awild-type mouse, and a knockout mouse of a protein having high-affinitycholine transporter activity of the present invention can be constructedby interclossing the heterozygous mice. It is possible to confirmwhether a knockout mouse of a protein having high-affinity cholinetransporter activity is constructed, for example, by isolating RNA fromthe mouse obtained by said method and examining it by Northern blotanalysis or the like, or by examining the expression of the mouse byWestern blot analysis or the like.

The transgenic mice of a protein having high-affinity cholinetransporter activity can be generated in following procedures. Atransgene is constructed by fusing chicken β-actin, mouse neurofilament,SV40 or other such promoters, and rabbit β-globin, SV40 or other suchpoly A or introns with cDNA that encodes a protein having high-affinitycholine transporter activity. The transgene is microinjected into thepronucleus of a fertilized egg of a mouse, and the obtained egg cell iscultured, then transplanted to the oviduct of a recipient mouse. Afterrearing up the recipient animal, baby mice that have the above-mentionedcDNA are selected from the mice born from the recipient animal. Thustransgenic mice can be generated. The baby mouse that has cDNA can beselected by extracting crude DNA from a tail or the like of a mouse,then carrying out methods like dot hybridization using the introducedgene that encodes a protein having high-affinity choline transporteractivity as a probe, PCR method using a specific primer, and the like.

In addition, cells being useful for gene therapy of Alzheimer's diseaseand the like can be prepared by using a whole or a part of a gene or DNAthat encodes a protein having high-affinity choline transporter activityof the present invention. As an example of a method for preparing thesecells of the present invention, a method wherein a whole or a part ofsaid gene or DNA of the present invention is introduced into a cellbeing deficient in the function of a gene that encodes a protein havinghigh-affinity choline transporter activity on its chromosome bytransfection or the like to obtain a cell having high-affinity cholinetransporter activity is exemplified. As the cell having high-affinitycholine transporter activity, in particular, it is preferable to use acell wherein said gene or DNA is integrated into a chromosome andhigh-affinity choline transporter activity is exhibited stably.

By using the above-mentioned gene or DNA that encodes a protein havinghigh-affinity choline transporter activity, a protein havinghigh-affinity choline transporter activity, a fusion protein created bycombining a protein having high-affinity choline transporter activityand a marker protein and/or a peptide tag, an antibody to a proteinhaving high-affinity choline transporter activity, a host cell whichcontains an expression system that can express a protein havinghigh-affinity choline transporter activity, a cell having high-affinitycholine transporter activity, or the like, it becomes possible to screena pharmaceutical material useful for the treatment of symptoms as inAlzheimer's disease or the like, in other words, a material thatpromotes or suppresses the activity or the expression of a high-affinitycholine transporter.

Examples of the screening method of the present invention are: a methodwherein the high-affinity choline transporter activity of theabove-mentioned protein having high-affinity choline transporteractivity of the present invention is measured/evaluated in the presenceof a subject material; a method wherein a cell membrane or a cell whichexpresses a protein having high-affinity choline transporter activity ofthe present invention is cultivated in vitro in the presence of asubject material, and the activity and/or the expression amount of aprotein having high-affinity choline transporter activity in the cell ismeasured/evaluated; and a method wherein a subject material isadministered to said non-human animal whose function of a gene encodinga protein having high-affinity choline transporter activity is deficientor overexpresses on its chromosome and/or a wild-type non-human animaland then the activity and/or the expression amount of a protein havinghigh-affinity choline transporter activity of the present invention ismeasured/evaluated. As said cell membrane or said cell, a cell such as aprimary cultured cell obtained from said non-human animal whose functionof a gene encoding a protein having high-affinity choline transporteractivity is deficient or overexpresses on its chromosome or a wild-typenon-human animal etc., a host cell containing an expression system whichcan express a protein having high-affinity choline transporter activityof the present invention, a cell having high-affinity cholinetransporter activity of the present invention, and cell membranes ofthese cells can be specifically exemplified.

The screening methods with said subject material and said protein havinghigh-affinity choline transporter activity are now specificallyexplained together with examples, but the screening methods of thepresent invention are not limited to these examples. Cells expressing aprotein having high-affinity choline transporter activity are culturedin the presence of a subject material, and the increase or the decreaseof a protein having high-affinity choline transporter activity expressedon the cell surface after a certain period of cultivation can beimmunochemically detected by ELISA or other such method with an antibodythat specifically combines to a protein having high-affinity cholinetransporter activity of the present invention, or can be evaluated byusing suppression or promotion of mRNA expression as an index. The mRNAcan be detected by methods such as DNA chip, Northern hybridization orthe like. Moreover, with a cell to which a gene wherein luciferase orother such reporter genes is linked to downstream of promoter of a genethat encodes high-affinity choline transporter is introduced, thesuppression or the promotion of the expression of a gene that encodes aprotein having high-affinity choline transporter activity induced by asubject material can be detected by using the activity of said reportergene as an index.

The present invention further relates a medical constituent being usedfor medical treatment for a patient who needs promotion of the activityor the expression of a protein having high-affinity choline transporter,or a medical constituent being used for medical treatment for a patientwho needs suppression of the activity or the expression of a proteinhaving high-affinity choline transporter, wherein the material containsa protein having high-affinity choline transporter activity, a materialwhich promotes the activity or the expression of a protein havinghigh-affinity choline transporter activity, or a material whichsuppresses activity or expression of a protein having high-affinitycholine transporter activity as an active component. As a protein havinghigh-affinity choline transporter activity is involved in manybiological functions including many pathological ones, it is expectedthat a compound that can stimulate a protein having high-affinitycholine transporter activity and a compound being able to inhibit thefunction of said protein can be used as pharmaceuticals.

As the material which promotes or suppresses the activity or theexpression of a protein having high-affinity choline transporteractivity, any material can be used as long as it binds to a proteinhaving high-affinity choline transporter activity, or works on a signaltransmitting molecule on upstream, and then promotes the activity or theexpression of a protein having high-affinity choline transporteractivity or inhibits/antagonizes the activity or the expression of theprotein by itself. Specific examples include an antibody, a ligand of aprotein having high-affinity choline transporter activity, a fragment ofsaid protein, and an oligonucleotide encoding said fragment, and thesematerials can be used as pharmaceuticals for treatment, prevention orthe like of symptoms observed in the case of Alzheimer's disease orother such diseases, but use of them is not limited to the aboveexamples.

The present invention also relates to a diagnostic method for diseasesrelating to the activity or the expression of a protein havinghigh-affinity choline transporter activity comprising a comparison of aDNA sequence encoding a protein having high-affinity choline transporteractivity in a sample with a DNA sequence encoding a protein havinghigh-affinity choline transporter activity of the present invention. Themutant type of DNA which encodes a protein having high-affinity cholinetransporter activity can be detected by finding gene-mutated individualsin DNA level, and this is useful for diagnosis of diseases caused byunderexpression, overexpression or mutant expression of a protein havinghigh-affinity choline transporter activity. Specific examples of asample of said detection include cells of trial subjects, for example,genomic DNA, RNA or cDNA obtained from biopsy of blood, urine, saliva,tissue or the like, however said sample is not limited to theseexamples. It is also possible to use said sample being amplified by PCRor other such methods. Deficiency and insertion mutation of basesequences can be detected by the size change of the amplified productobserved in comparison with normal genotype, and point mutation can beidentified by hybridizing amplified DNA with a labeled gene that encodesa protein having high-affinity choline transporter activity. Thus,diagnosis or judgement of symptoms observed in the case of Alzheimer'sdisease or other such diseases can be made by detecting the mutation ofa gene that encodes a protein having high-affinity choline transporteractivity.

The present invention further relates to a diagnostic probe for diseasesshowing symptoms similar to those of Alzheimer's disease or the likecomprising a whole or a part of an antisense chain of DNA or RNAencoding a protein having high-affinity choline transporter activity,and a diagnostic drug for diseases showing symptoms similar to those ofAlzheimer's disease containing the diagnostic probe and/or an antibodywhich specifically binds to a protein having high-affinity cholinetransporter activity of the present invention. Said diagnostic probe isnot limited in particular, as long as it comprises a whole or a part ofan antisense chain of DNA (cDNA) or RNA (cRNA) encoding a protein havinghigh-affinity choline transporter activity and being long enough to be aprobe (at least 20 bases). In order to make a diagnostic drug forsymptoms similar to those of Alzheimer's disease containing said probeand/or an antibody which specifically binds to a protein havinghigh-affinity choline transporter activity of the present invention asactive components, it is preferable to dissolve said probe into anappropriate buffer or sterilizing water for preventing said probe fromdecomposition. Further, it is also possible to diagnose diseases showingsymptoms similar to those of Alzheimer's disease by methods using thesediagnostic drugs, such as immunostaining (Dev. Biol. 170, 207-222, 1995,J. Neurobiol. 29, 1-17, 1996), in situ hybridization (J. Neurobiol. 29,1-17, 1996), in situ PCR or the like.

Experimental methods or the like of the above-mentioned variousexperiments will now be explained in more detail below.

(Cloning of High-Affinity Choline Transporter cDNA)

The candidate cDNA of nematode high-affinity choline transporter wasisolated from poly (A)+RNA of nematode mixture from various stages inthe development by reverse transcription PCR and 3′ RACE. Marathon™ cDNAAmplification Kit (Clontech) was used according to its protocol. Aprimer for sense direction of PCR was designed at a provisionaltranslation initiating point of a predicted gene based on a DNA basesequence obtained from C. elegans genomic project. The amplified PCRproduct was subcloned into Nco I (smoothing) site and Not I site of amodified pSPUTK vector (Stratagene), and the base sequence of insertedDNA was determined. CHT1 cDNA of rat was isolated from rat spinal cordcDNA library by using GeneTrapper cDNA Positive Selection System (GIBCOBio-Rad Laboratory: GIBCO BRL) according to its protocol. The primerused was designed from the base sequence of a cDNA fragment obtained bydegenerated PCR. The obtained cDNA clones were analyzed. Among them,positive clones were selected and subcloned into pSPUTK vector andpcDNA3.1+ vector (Invitrogen Corporation).

(Expression in Oocytes of Xenopus)

In the presence of cap analog, cRNA was synthesized in vitro with SP6 orT7 RNA polymerase. 20 to 30 ng capped RNA was microinjected into oocytes(stage V to VI) of Xenopus. The uptake was measured in basically samemanner as described previously (Nature 360,467-471, 1992). Two or threedays after the injection of RNA, choline uptake was conducted for 30 to60 min. with oocytes (6 to 8) in 0.75 ml standard solution (0.01 to 1 μM[³H]-choline, 100 mM NaCl, 2 mM KCl, 1 mM MgCl₂, 1 mM CaCl₂, 10 mMHEPES, 5 mM Tris: pH 7.4). The oocytes completing uptake weresolubilized with 10% SDS, and the amount of [³H] was measured by aliquid scintillation counter.

(GFP Expression Construct)

The transcriptional fusion construct of cho-1::gfp was constructed byPCR in same manner as described previously (Gene 212, 127-135, 1998). Agene that encodes a green fluorescent protein (GFP) located ondownstream of a nuclear localization signal sequence (NLS) was insertedinto a position 3 residues downstream of cho-1 translation initiatingpoint so that the reading frame was fitted. NLS and gfp gene wereamplified from pPD104.53 vector. In order to prepare 5.1 kb upstreamregion of cho-1 translation initiating point, a PCR primer beingdesigned to encompass the first 3 amino acid residues of cho-1 was used.By the same method as previously described (EMBO J. 10, 3959-3970,1991), rol-6 (su1006) marker and generated DNA were injected into gonadsof a nematode simultaneously.

(Northern Blot Analysis)

6 μg poly(A)+RNA prepared from various tissues of rats was separated byformaldehyde-agarose electrophoresis, and transferred to a nylonmembrane, then hybridized with CHT1 cDNA fragment being labeled with[³²P ] by random prime method in hybridization solution (solutioncontaining the final concentration of 50% formamido, 5×SSPE,5×Denhardt's solution, 0.5% SDS, 100 μg/ml salmon sperm DNA) at 42° C.for 16 hours. The nylon membrane was washed under final condition(0.1×SSPE, 0.1% SDS: 65° C.), and then autoradiography was conducted for7 days together with an enhancing screen.

(In Situ Hybridization)

The transcript of an antisense labeled with digoxigenin was synthesizedin vitro. Alkaline hydrolysis was repeated for the transcripts untiltheir mean length was prepared to be 200 to 400 b. Cryostat sections offresh frozen tissue (10 to 20 μm) were used. Hybridization was conductedwith labeled cRNA probe (about 1 μg/ml) dissolved in 1×Denhardt'ssolution [solution containing the final concentration of 50 mM Tris-HCl(pH 8.0), 2.5 mM EDTA, 0.3 M NaCl, 50% formamido, 10% dextran sulphate,1 mg/ml E. coli tRNA] at 45° C. for 20 hours. Then the sections werewashed twice in 2×SSC/50% formamido and once in 1×SSC/50% formamido, at45° C. respectively. The hybridized probe was visualized by usinganti-digoxigenin Fab fragment (Boehringer-Mannheim) and NBT/BCIPsubstrate. The sections were brought into reaction in substrate solutionfor 24 to 48 hours.

(Binding Assay)

[³H] hemicholinium-3 (HC3; 128Ci/mmol) was obtained from NEN LifeScience Products. Either pcDNA3.1-CHT1 or pcDNA3.1 was transientlyexpressed in COS7 cells respectively. TransFast Reagent (Promega) wasintroduced and used according to the protocol. Membranes were preparedby following steps: homogenizing cells in 0.32 M sucrose; centrifugingthe cells for 1 hour at 200,000 g; and suspending the precipitate.Binding assay was conducted in basically same manner as describedpreviously. Specific binding amount was calculated by subtractingnon-specific binding amount determined in the presence of 10 μM HC3 fromthe whole binding amount. The Kd value was figured out by analyzingspecific [³H] HC3 binding amount from data of saturation binding assaywith nonlinear approximation.

INDUSTRIAL APPLICABILITY

The present invention makes it possible to provide a protein havinghigh-affinity choline transporter activity, which is physiologicallyimportant, and gene DNA encoding said protein. In addition, by using thesaid protein and gene DNA, it becomes possible to screen materials beinguseful for prevention or treatment of Alzheimer's disease, and toprepare cells being useful for gene therapy.

1. An isolated nucleic acid which encodes a protein comprising an aminoacid sequence represented by Seq. ID No.
 6. 2. An isolated proteincomprising an amino acid sequence represented by Seq. ID No.
 6. 3. Afusion protein wherein the protein according to claim 2 and a markerprotein and/or peptide tag are bound together.
 4. An isolated host cellcomprising an expression system capable of expressing the proteinaccording to claim 2.